Cell division plays a crucial role during all phases of plant development. The continuation of organogenesis and growth responses to a changing environment requires precise spatial, temporal and developmental regulation of cell division activity in meristems (and in cells with the capability to form new meristems such as in lateral root formation). Such control of cell division is also important in organs themselves (i.e. separate from meristems per se), for example, in leaf expansion, secondary growth, and endoreduplication.
A complex network controls cell proliferation in eukaryotes. Various regulatory pathways communicate environmental constraints, such as nutrient availability, mitogenic signals such as growth factors or hormones, or developmental cues such as the transition from vegetative to reproductive stages. Ultimately, these regulatory pathways control the timing, frequency (rate), plane and position of cell divisions.
Plants have unique developmental features that distinguish them from other eukaryotes. Plant cells do not migrate, and thus only cell division, expansion and programmed cell death determine morphogenesis. Organs are formed throughout the entire life span of the plant from specialized regions called meristems. In addition, many differentiated cells have the potential to both dedifferentiate and to reenter the cell cycle. There are also numerous examples of plant cell types that undergo endoreduplication, a process involving nuclear multiplication without cytokinesis. The study of plant cell cycle control genes is expected to contribute to the understanding of these unique phenomena. O. Shaul et al., Regulation of Cell Division in Arabidopsis, Critical Reviews in Plant Sciences 15 (2):97–112 (1996).
Current transformation technology provides an opportunity to engineer plants with desired traits. Major advances in plant transformation have occurred over the last few years. However, in many major crop plants, serious genotype limitations still exist. Transformation of some agronomically important crop plants continues to be both difficult and time consuming. For example, it is difficult to obtain a culture response from some maize varieties. Typically, a suitable culture response has been obtained by optimizing medium components and/or explant material and source. This has led to success in some, but not all, genotypes. While, transformation of model genotypes is efficient, the process of introgressing transgenes into production inbreds is laborious, expensive and time consuming. It would save considerable time and money if genes could be introduced into and evaluated directly in commercial hybrids.
Current methods for genetic engineering in maize require a specific cell type as the recipient of new DNA. These cells are found in relatively undifferentiated, rapidly growing callus cells or on the scutellar surface of the immature embryo (which gives rise to callus). Irrespective of the delivery method currently used, DNA is introduced into literally thousands of cells, yet transformants are recovered at frequencies of 10−5 relative to transiently-expressing cells. Exacerbating this problem, the trauma that accompanies DNA introduction directs recipient cells into cell cycle arrest and accumulating evidence suggests that many of these cells are directed into apoptosis or programmed cell death. (Reference Bowen et al., Tucson International Mol. Biol. Meetings). Therefore, it would be desirable to provide improved methods capable of increasing transformation efficiency by reducing cell death in a number of cell types.
Despite the number of identified plant diseases and stresses, the understanding of the molecular and cellular events that are responsible for plant disease and stress resistance remains rudimentary. This is especially true of the events controlling the earliest steps of active plant defense, recognition of a potential pathogen and transfer of the cognitive signal throughout the cell, surrounding tissue and cell death within that tissue.
Diseases are particularly destructive processes resulting from specific causes and characterized by specific symptoms. Generally the symptoms can be related to a specific cause, usually a pathogenic organism. In plants, a variety of pathogenic organisms cause a wide variety of disease symptoms. It would therefore be valuable to develop new methods that contribute to the increase in crop yield to protect plants against cell death associated with pathogen and stress.
In spite of increases in yield and harvested area worldwide, it is predicted that over the next ten years, meeting the demand for corn will require an additional 20% increase over current production (Dowswell, C. R., Paliwal, R. L., Cantrell, R. P. (1996) Maize in the Third World, Westview Press, Boulder, Colo.).
The components most often associated with maize productivity are grain yield or whole-plant harvest for animal feed (in the forms of silage, fodder, or stover). Thus the relative growth of the vegetative or reproductive organs might be increased or decreased, depending on the ultimate use of the crop. Whether the whole plant or the ear are harvested, overall yield will depend strongly on vigor and growth rate. It would therefore be valuable to develop new methods that contribute to the increase in crop yield.